The present invention relates generally to Frequency Modulated (FM) systems, and in particular to a system and method providing an improved squelch system for filtering weak signals within an FM system.
A primary function for an FM carrier squelch system is to provide for filtering out weak signals, wherein the determination of whether a signal is weak or not generally depends on the Radio Frequency (RF) level of a received carrier with respect to noise power. In order to provide users the ability to filter out poor signals, it is generally necessary determine the signal-to-noise ratio of the received signal.
A received FM modulated signal generally takes the form of:
y(t)=Ac cos[wct+c(t)]
xcex8c(t)=kf∫m(t)∂t
wherein,
Ac is the amplitude of the carrier, wc is the carrier frequency, kf is the frequency deviation factor and m(t) is the message signal modulated onto the carrier. This signal is mixed down into in-phase and quadrature phase components for demodulation. An FM discriminator output may be given by:       x    ⁢          (      t      )        =                    ∂                  ∂          t                    ⁢                        θ          c                ⁢                  (          t          )                      =                            ∂                      ∂            t                          ⁡                  [                                    tan                              -                1                                      ⁢                          (                              Q                I                            )                                ]                    =                        ∂                      ∂            t                          ⁡                  [                                    tan                              -                1                                      ⁢                          (                                                                                          A                      c                                        ⁢                    sin                    ⁢                                          xe2x80x83                                        ⁢                                                                  θ                        c                                            ⁢                                              (                        t                        )                                                                              +                                      N                    q                                                                                                              A                      c                                        ⁢                    cos                    ⁢                                          xe2x80x83                                        ⁢                                                                  θ                        c                                            ⁢                                              (                        t                        )                                                                              +                                      N                    i                                                              )                                ]                    
wherein I and Q are an in-phase and quadrature phase channel and Ni and Nq are the associated independent noise sources. Thus, an FM discriminator output (and an input to a squelch system) is a function not only of the carrier strength and noise environment, but also of the message signal and frequency deviation factor.
In February 1993, The Telecommunications Industry Association published TIA/EIA-603. This document includes a set of standards for Land Mobile FM or PM communications equipment, measurement and performance. Two tests, in particular, relate to squelch: Audio Squelch Sensitivity and Squelch Blocking. Both tests refer to a signal-to-noise ratio called SINAD, which is given by:       SINAD    ⁡          (      dB      )        =      20    ·                  log        10            ⁡              (                              Signal            +            Noise            +            Distortion                                Noise            +            Distortion                          )            
The audio squelch sensitivity of a receiver is a minimum signal level from a standard input signal source which opens the receiver squelch. The standard input signal is a 1 kHz frequency sinusoid at 60% of the maximum system deviation. The squelch setting should be set to a position that requires the smallest input signal to produce an un-muted audio output while the RF power is slowly increased until the radio un-mutes. The SINAD that corresponds to this RF level is called the threshold squelch sensitivity. The squelch setting should then be set to the position that requires the largest input signal to produce an un-muted audio output wherein the RF power is slowly increased until the radio un-mutes. The SINAD that corresponds to this RF level is called the tight squelch sensitivity.
Squelch blocking refers to the tendency of a receiver squelch circuit to close in the presence of modulation of the input signal. A user-controlled squelch setting should thus be adjusted to the position that requires the largest input signal to produce an un-muted audio output. For example, an input signal 12 dB above the measured tight squelch sensitivity may be applied. The deviation is increased to the rated system deviation and the modulation frequency is varied slowly from 300 Hz to 3 kHz. The receiver remains un-muted for the duration of the test.
It is noted that the EIA/TIA specifications may ignore a fundamental difficulty in a squelch system, however. When the audio sensitivity test standardizes the input to a sinusoid of frequency 1 kHz and at a deviation of 60% system maximum, for example, the standard does not fully consider that x(t) is dependent upon the message signal. The squelch blocking test is a bit more flexible in that a modulated signal is varied during the test, however, a more useful test would utilize a spectrally rich signal such as speech. Thus, while EIA/TIA standards are a good starting point, they are by no means complete.
Other squelch systems address the dependence of x(t) on the message signal within a single system. Most analog radios have a much wider spectrum and view out-of-band energy as a measure of the noise power. Conventional algorithms, for example, may compare the out-of-band energy with a reference level computed by a curve fitting polynomial. The curve fitting polynomial may characterize the noise power as a function of modulation in order that the algorithm may compensate for signals modulated onto the carrier. Unfortunately, these algorithms are both computationally intensive and may fail the squelch blocking test, described above. Consequently, there is an unsolved need for an improved squelch system and methodology providing higher system performance and improved test standards conformance.